Patent GB-721907-A (GB)
Title: Methods of forming thermally dehydrogenated products of hydrocarbon polymers, and the products so produced, particularly carbon granules for microphones
IPC: C01B31/00
Filed: 1952-04-25
Abstract: Bodies which are predominantly carbon are prepared by thermally dehydrogenating in situ by heating to a temperature of at least 200 DEG C. a solid or substantially solid polymer substance having an intrinsic vapour pressure not greater than 10-4 centimetres of mercury at 500 DEG C., which is formed by the additional polymerization of an unsaturated hydrocarbon material and which does not contain any element other than hydrogen, which in its uncombined state is normally gaseous, the heating being continued until the hydrogen content of the substance has been reduced to not more than 6 per cent. by weight of the resulting product. Suitable polymers and the requisite intrinsic vapour pressure are firstly those containing at least 50 per cent. by weight of one or more monomers having a functionality of at least 6 and secondly polymers formed by the polymerization of monomers having a functionality of at least 2.4 and which are first heated in an oxidizing atmosphere before they are subsequently dehydrogenated. Suitable monomers are those containing olefinic or acetylenic unsaturated bonds such as divinyl benzene, trivinyl benzene, divinyl naphthalene, vinyl acetylene, divinyl acetylene, bis (p-vinyl phenyl) methane, vinyl butadienyl acetylene, lycopene or beta carotene or mixtures thereof.  Co-polymers preferably contain at least 20 per cent. by weight of a divinyl aromatic hydrocarbon. Polymerization of the monomers is effected in the conventional manner, the monomers or partially polymerized material being formed into the desired shape.  Polymerization may be accomplished by adding 1 per cent. by weight of benzoyl peroxide and heating to the temperature between 60 and 150 DEG C.  In addition to this and other polymerization catalysts, promoters such as cobalt or iron naphthenate, dimethyl aniline, ethyl, butyl, or dodecyl mercaptan or axo bis-butyronitrile may be used.  In the formation of polymer spheres the material to be polymerized is agitated by rapid stirring in a body of non-solvent suspension liquid and the entire system is maintained at polymerizing temperatures until rigid spheres are produced.  Similarly, polymer films may be deposited on surfaces such as ceramic, glasses, crystals, or metals in the form of rods, wires, spheres, or tubes by applying the liquid monomer to the surface and maintaining it at polymerizing temperature or by employing partially polymerized material dissolved in a volatile solvent.  The resistance of the polymer to depolymerization on further heating is increased by heating the polymer in air from 75 to 300 DEG C. prior to pyrolysis in a non-oxidizing atmosphere, pyrolysis temperatures rising to 1,300 DEG  C.  The tendency of polymer bodies to work on pyrolysis may be avoided by the application of pressures up to 1,000 p.s.i. during pyrolysis.  The process may also be applied to polymers containing hydrogen carbon and silcon, the latter constituting normally between 0.1 and 20 per cent. by weight.  Typical monomers which may be used alone or admixed with each other or with other polymerizable hydrocarbons are polyallyl silanes and silyl styrenes.  Silyl styrenes are preferable copolymerized with at least 20 per cent. of a substance such as divinyl or trivinyl benzene, vinyl acetylene, divinyl acetylene, divinyl naphthalene, vinyl butadienyl acetylene, or trivinyl benzene. Thus spheres for electrical use are prepared by pyrolysis of a copolymer of 10 parts by weight of trimethyl silyl styrene with 50 parts of divinyl benzene and 40 parts of ethyl vinyl benzene.  The spheres after formation are air baked at 250 DEG C. for 16 hours followed by dehydrogenation in nitrogen at temperatures increasing to 960 DEG C.; the product contains 1.2 per cent. silicon.  Spheres formed of polymer carbon may be used for microphone granules, in which case up to 20 per cent. of the carbon may be replaced by silicon, boron, phosphorus, silver, titanium, aluminium, germanium, bismuth or tin, such elements being introduced by forming the carbon from a modified hydrocarbon polymer containing these elements in the network thereof or in side groups or chains.  For microphone granules, the polymer carbon may constitute the core of the granule which is subsequently coated with pyrolytic carbon deposited from a hydrocarbon gas or the polymer carbon may be deposited on a core of quartz, silica gel or pyrolytic carbon. Spherical bodies of polymer carbon may also be used as adsorbents for liquids and gases, in gas detectors where the constant resistance between the spheres in the presence of certain gases varies, or as ball-bearings for instruments and watches.  The absolute density of the polymer carbon in bodies such as spheres is slightly greater than 2, the carbon occupying about half of the apparent volume.